1. Field of the Invention
The present invention relates to a solid state imager, and in particular to a solid state imager characterized in that, within each pixel thereof, electric charges generated by an incident light are multiplied in avalanche multiplication mode.
2. Description of the Prior Art
For the pixel-amplification type solid state imager, wherein amplification is conducted by an amplifier adjacent to each pixel during read out time, AMI, CMD, SIT, BASIS and etc. have so far been proposed.
They have some disadvantages as follows:
1. An enhanced number of components are needed to make a pixel.
2. Because a current is read out from a pixel, a restricted read-out time results in a restricted amplification gain.
3. The sweeping-out of signal charges is not enabled in the same way as done in the current imagers in a high-speed electronic shutter mode through vertical overflow drain.
4. Primarily due to the manufacturing process, there exists inherently a fixed pattern noise caused by the fluctuation of the gate width of the amplifier.
Against the pixel-amplification type solid state imager, there are pick-up tubes wherein signal charges generated by an optical-to-electrical conversion are in situ amplified.
For a High Definition Television (HDTV)-use high sensitive pick-up tube, HARP tube was proposed which utilizes avalanche phenomena for amplification. However, the device has the following disadvantages:
5. A big reduction in size is difficult.
6. A voltage higher than 180 V is needed to drive the device.
7. The device is fragile against mechanical vibration or shock.
8. The device is apt to be distorted in images and to be easily effected by terrestrial magnetism.
Although these disadvantages are recognized, it has been confirmed that the device has a gain as high as 1000 times and that it exhibits lower noise probably stemming from avalanche multiplication run on charge-storage-mode operation.